1. Field of the Invention
The present invention relates to a control apparatus of a vibration-type actuator and a control method of a vibration-type actuator.
2. Description of the Related Art
Actuators are conventionally proposed which subject a given mass point on a plate-like vibrator to elliptical motion (driving member) to drive a driving member.
As a basic configuration of a vibration-type actuator with a plate-like vibrator, such a configuration as illustrated in Japanese Patent Application Laid-Open No. 2004-320846 is known. FIG. 8A is a perspective view illustrating an example of the external basic configuration of the vibration-type actuator in Japanese Patent Application Laid-Open No. 2004-320846.
As illustrated in FIG. 8A, a vibrator in the vibration-type actuator includes an elastic member 4 formed of a metal material shaped like a rectangular plate. The elastic member 4 includes a piezoelectric element (electromechanical energy transducer) 5 joined to a back surface thereof. A plurality of protrusions 6 are provided on a top surface of the elastic member 4 at respective predetermined positions.
According to this configuration, applying an AC voltage to the piezoelectric element 5 allows simultaneous generation of secondary bending vibration in a long side direction of the elastic member 4 and primary bending vibration in a short side direction of the elastic member 4. This excites elliptical motion in the protrusions 6.
Then, the driving member 7 is brought into contact with the top portions (contact portions) of the protrusions 6 under pressure and then linearly driven by elliptical motion of the protrusions 6. That is, the protrusions 6 act as a drive unit for the vibrator.
FIG. 8B is a schematic diagram illustrating an example of a polarization area of the piezoelectric element 5 in the vibration-type actuator illustrated in FIG. 8A.
Furthermore, FIGS. 9A and 9B are perspective views illustrating a vibration mode of the elastic unit 4. FIG. 9C is a diagram illustrating elliptical motion excited in the protrusions 6 of the elastic unit 4.
The piezoelectric element 5 is subjected to a polarization process and includes two electrodes A1 and A2, as illustrated in FIG. 8B.
AC voltages V1 and V2 in phase with each other are applied to the two electrodes A1 and A2, respectively, to excite the rectangular elastic unit 4 into primary bending movement with two nodes extending in a direction parallel to the long side direction. This corresponds to a first vibration mode illustrated in FIG. 9A.
Furthermore, the AC voltages V1 and V2 out of phase with each other are applied to the two electrodes A1 and A2, respectively, to excite the rectangular elastic unit 4 into secondary bending movement with three nodes extending in a direction parallel to the short side direction. This corresponds to a second vibration mode illustrated in FIG. 9B.
Then, the first vibration mode and the second vibration mode are combined together to excite elliptical motion in the protrusions 6. At this time, when brought into contact with the protrusions 6 under pressure, the driving member can be linearly driven.
Here, the first vibration mode illustrated in FIG. 9A allows activation of an amplitude (hereinafter referred to as a Z-axis amplitude) displaced in a direction perpendicular to the surface of the contact portion (hereinafter referred to as the contact surface brought into contact with the driving member under pressure in the protrusions 6.
Furthermore, the second vibration mode illustrated in FIG. 9B allows an amplitude (hereinafter an X-axis amplitude) displaced in a direction parallel to the contact surface to be excited in the protrusions 6.
Combination of the first vibration mode and the second vibration mode allows elliptical motion to be excited in a predetermined one of the protrusions 6 as illustrated in FIG. 9C. The ratio in magnitude between the Z-axis amplitude and the X-axis amplitude is hereinafter referred to as ellipticity of elliptical motion.
FIG. 10A is a graph illustrating the amplitudes in the first vibration mode and the second vibration mode observed when the difference in phase between the two-phase voltages V1 and V2 is changed between −180 degrees and 180 degrees.
When the difference in phase between the two-phase AC voltages V1 and V2 applied to the respective two electrodes A1 and A2 of the polarized piezoelectric element is changed between −180 degrees and 180 degrees, the amplitudes in the first vibration mode and the second vibration mode (P2) are as illustrated by P1 and P2 in FIG. 10A, respectively.
In FIG. 10A, the axis of abscissas indicates the phase difference. The axis of ordinate indicates the amplitudes in the first amplitude mode and in the second amplitude mode.
Combination of the first vibration mode and the second vibration mode allows elliptical motion to be excited in the protrusions 6. Changing the phase difference between the AC voltages V1 and V2 to be applied allows adjustment of ellipticity of elliptical motion excited in the predetermined protrusion 6.
FIG. 10A illustrates, in the lower part thereof, elliptical shapes corresponding to the phase differences on the axis of abscissas. The direction of driving by the vibration-type actuator, which provides linear driving, can be switched by switching between the positive sign and negative sign of the phase difference between the AC voltages V1 and V2.
Moreover, the direction and speed of driving can be consecutively changed by consecutively changing the phase difference starting with any value, with the sign appropriately changed between the positive one and the negative one (for example, consecutively changing the phase difference between 90 degrees and −90 degrees, with the sign appropriately changed between the positive one and the negative one).
Concerning the driving speed, the following phenomenon (which is called a cliff drop phenomenon) occurs as illustrated in FIG. 10B. The driving speed peaks at the resonant frequency and decreases slowly on a higher frequency side of the resonant frequency, while decreasing rapidly on a lower frequency side of the resonant frequency.
Furthermore, as is generally known, the speed can be increased by setting the frequency of the AC voltage applied to the piezoelectric element closer to the resonant frequency. The speed can be reduced by setting the frequency of the applied AC voltage further away from the resonant frequency.
As such a vibration-type actuator, an apparatus can be provided which exerts a driving force increased using a plurality of vibrators.
However, when the vibration-type actuator is configured to drive the driving member, using a plurality of vibrators, the following problem may occur.
When a common frequency is applied to each of the plurality of vibrators in order to simplify the circuit configuration of a control apparatus of the vibration-type actuator, the vibration-type actuator operates unstably if the resonant frequency varies among the vibrators. Thus, the vibration-type actuator needs to drive the object without using frequency regions corresponding to the unstable operation.
In view of the above-described problem, an object of the present invention is to provide a control apparatus of a vibration-type actuator and a control method of a vibration-type actuator in which the vibration-type actuator configured to drive the driving member using a plurality of vibrators can drive the object stably even with a variation in resonant frequency among the vibrators.